This invention relates generally to borehole formation evaluation instrumentation. More particularly, this invention relates to a method for correcting errors in formation resistivity logs derived from an electromagnetic propagation based borehole formation evaluation instrument used primarily in oil and gas well drilling applications.
Borehole formation evaluation tools are known which measure phase and/or amplitude of electromagnetic waves to determine an electrical property (such as resistivity or permittivity) of a section of a borehole. Typically, the existing tools used for this application are composed of one or more transmitting antennas spaced from one or more pairs of receiving antennas. An electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes by the two receiving antennas. In a resistivity measuring tool, magnetic dipoles are employed which operate in the mf and lower hf spectrum. In contrast, permittivity tools utilize electric dipoles in the VHF or UHF ranges.
In a known resistivity sensor of the type hereinabove discussed which is used by Teleco Oilfield Services, Inc., assignee of the present application, the resistivity sensor measures both phase difference and amplitude ratio to provide two resistivities with different depths of investigation. A signal received in a first receiving antenna is shifted in phase and its amplitude will be less than the signal received in a second receiving antenna. Resistivities are then derived from both the phase difference (R.sub.pd) and the amplitude ratio (R.sub.ar) of the received signals. This differential measurement is primarily responsive to the formation opposite the receiving antennas and is less sensitive to the borehole and/or variations in the transmitted signal as in prior art sensing devices. An example of a formation evaluation instrument of this type is described in FIGS. 1 and 2 of U.S. Pat. No. 5,001,675 which is assigned to the assignee hereof and fully incorporated herein by reference.
While well suited for its intended purposes, a problem with existing electromagnetic propagation sensors of the type described herein consists of errors present in the respective R.sub.pd and R.sub.ar curves which make up the formation resistivity log. Theoretically, the two resistivity curves R.sub.ar and R.sub.pd should be coincident or at least substantially coincident. However, under typical measurement conditions, the two resistivity curves R.sub.ar and R.sub.pd deviate from each other beyond what would be theoretically expected with separations or gaps resulting between the two curves. These deviations are due to dielectric effects which are accounted for in the transformation from phase difference and amplitude ratio to the resistivity values R.sub.ar and R.sub.pd. These separations represent error in the resistivity curves R.sub.ar and R.sub.pd and may result in formation resistivity logs having a less than desirable accuracy.
U.S. Pat. No. 4,899,112 to Clark et al describes a method for correcting this error. The Clark et al patent uses a look-up table derived from measured core data (see FIG. 12 of Clark et al) and an iterative method (as described in FIG. 8 of Clark et al) to derive a correction to the resistivity curves R.sub.ar and R.sub.pd. The iterative method of Clark et al uses the core data to derive a correlation between the dielectric permittivity and conductivity of earth formations. This correlation is then used to make the transformation from phase difference or amplitude ratio to resistivity. Unfortunately, this method is limited by the accuracy of the core data. It is well known that accurate core data (at operating frequencies of 2 MHz) is difficult to obtain. Thus, the corrective method of Clark et al is itself subject to inaccuracy and error.